Increasing concentrations of Carbon Dioxide (CO2) in the Earth's atmosphere have recently caused concerns, as the present concentrations are substantial at 400 p.p.m. and increasing at a rate of 4 p.p.m. per annum. A major factor contributing to such increase is the anthropogenic oxidation of carbonaceous fossil fuels, for example coal, oil and gas. The World presently consumes circa 100 million barrels of oil per day.
There have been recent initiatives to employ more renewable energy systems, for example wind turbines and tidal power generating systems, as well as sequestration of Carbon Dioxide (CO2) from flue-gases emitted from coal-burning electricity generating facilities. Thus, primary initiatives involve utilizing energy efficient technologies, increasing reliance on renewable sources, and developing technologies for long term storage of Carbon Dioxide (CO2) emissions. The latter technology field is known as Carbon Dioxide (CO2) sequestration.
There have been significant developments in Carbon Dioxide (CO2) sequestration in recent years, and Carbon Dioxide sequestration technologies have aroused considerable interest among governments, industries and scientific communities. Earlier methods of Carbon Dioxide (CO2) sequestration suffered from various drawbacks such as risk of water contamination, and a lack of suitable storage spaces for receiving sequestrated Carbon Dioxide (CO2); these drawbacks are familiar to a person skilled in the art.
There has been considerable interest in mineral Carbonation technologies to address aforementioned problems associated with known Carbon Dioxide (CO2) sequestration. A Masters thesis by Mabell Delgado Torróntegui at ETH in Switzerland, “Assessing the Mineral carbonation science and technology” (2010), provides an overview of contemporary research in this field of Carbon Dioxide (CO2) sequestration technology. A key principle of mineral Carbonation is also known as “mineral sequestration technology”, wherein sequestration of Carbon Dioxide (CO2) is achieved by capturing Carbon Dioxide (CO2) in a form of stable mineral Carbonates. Such sequestration employs a process which is an exothermic reaction of a metal Oxide and Carbon Dioxide (CO2) to form stable Carbonate materials as provided in a reaction formula (1):MO+CO2=>MCO3+Heat  (1)wherein M is a metal, preferably an alkaline earth metal such as Calcium or Magnesium.
Most suitable and naturally abundant sources of these metal Oxides are Magnesium or Calcium Silicate minerals such as Olivine, Wollastonite, and Serpentine. The Carbonation reactions of these minerals are as follows:
Olivine:Mg2SiO4+2CO2=>2MgCO3+SiO2+89 kJ mol-1 CO2  (2)
Serpentine:Mg3Si2O5(OH)4+3CO2=>3MgCO3+2SiO2+2H2O+64 kJ mol-1 CO2  (3)
Wollastonite:CaSiO3+CO2=>CaCO3+SiO2+90 kJ mol-1 CO2  (4)
Although the above reactions (2) to (4) are thermodynamically favourable, the reactions in nature have, however, relatively slow reaction rates in a geologic time scale, and are unsuitable for industrial processes. Efforts have been made to try to accelerate these reactions. However, the efforts suffer from various limitations, such as energy wastage and a high cost of mining and transporting large amounts of rock, as well as industrial and environmental inefficiencies. Moreover, the mineral Silicates are not easily obtainable in suitable quantities and formats for allowing satisfactory mineral Carbonation to be achieved.
In a published U.S. Pat. No. 5,604,787B2 (MAROTO-VALER), “Process for sequestering Carbon Dioxide and Sulphur Dioxide”, there is described a method of reacting a Silicate-based material with an acid to form a suspension, which is then combined with Carbon Dioxide to produce a metal salt, silica and regenerating acid in solution. This method has drawbacks of being environmentally harsh and inefficient. Moreover, similar problems with the approach are described in US patent application 2004126192A1 (SHELL INTERNATIONAL RESEARCH), “Process for Removal and Capture of Carbon Dioxide from Flue Gases”.
Although, the prior art disclosures have been able to address some of the problems of mineral Carbonation through their indirect sequestration processes, there are several remaining problems which have not yet been resolved. Such remaining problems pertain to industrial scalability, environmental efficiency, and cost.
Lately, research effort has focused upon Carbon Dioxide (CO2) sequestration by ‘direct’ Carbonation of Olivine or Serpentine. In these recent methods, Carbon Dioxide (CO2) is sequestered without acid pre-treatment of Silicate feedstock. An Olivine reaction is:Mg2SiO4+2CO2→2MgCO3+SiO2  (5)
Moreover, for Serpentine, a corresponding reaction is:Mg3Si2O5(OH)4+3CO2→3MgCO3+2SiO2+2H2O  (6)
Experiments to determine the kinetics of these reactions (5) and (6) have shown that such reactions also suffer from poor energy efficiency and a high cost when up-scaled to an industrial plant. In order to accelerate the reactions (5) and (6), high temperatures in a range of 600° C. to 650° C. are required. In a fuel-fired industrial power plant, attainment of such high temperatures would translate to a requirement of approximately 200 kW-h of electricity per tonne of Serpentine feedstock. Moreover, with a fossil fuel containing 1 tonne of Carbon, nearly 3.7 tonnes of Carbon Dioxide (CO2) is produced. Each tonne of Carbon Dioxide (CO2) consumes more than 2 tonnes of Serpentine during Carbonation. Combined with a pre-capture step to separate and pressurise CO2 from flue gas, the power required for Serpentine dehydroxylation is around 20-30% of total power output from such fuel-fired industrial power plant. All these considerations lead to a huge energy penalty threatening the economic feasibility of this sequestration process.
In a published U.S. Pat. No. 8,114,374B2 (BLENCOE), “Carbonation of metal Silicates for long-term CO2 sequestration”, there is described a method (hereinafter “Blencoe's method”) of reacting a Silicate with an alkali metal hydroxide in an aqueous solution. The reaction with Carbon Dioxide (CO2) is then used to carbonate the metal formerly contained in the metal Silicate. This method has drawbacks of inefficiencies and poor overall Carbon capture properties. Blencoe's method comprises three steps:    (1) reacting a metal silicate with a caustic alkali-metal hydroxide to produce a hydroxide of the metal formerly contained in the silicate;    (2) reacting carbon dioxide with at least one of a caustic alkali-metal hydroxide and an alkali-metal silicate to produce at least one of an alkali-metal carbonate and an alkali-metal bicarbonate; and    (3) reacting the metal hydroxide product of step (a) with at least one of the alkali-metal carbonate and the alkali-metal bicarbonate produced in step (b) to produce a carbonate of the metal formerly contained in the metal silicate of step (a).
In Blencoe's first step (1), the alkali-metal hydroxide is in aqueous solution which limits the maximum reaction temperature to the vapour pressure of the aqueous solution at the pressure under which the reaction is carried out. A low reaction temperature limits the reaction rate at atmospheric pressure, while use of a pressurised vessel increases process costs. Alkali-metal hydroxides, such as NaOH and KOH, used in steps (1) and (2) are energy-intensive and expensive chemicals to manufacture. In Blencoe's second step (1), alkali-metal hydroxide, additional to that used in step (1), is used to react with Carbon Dioxide. In Blencoe's second and third steps (2) and (3), an elevated pressure is required, namely having implied costs of a pressure vessel, in the range between the vapour pressure of water at the reaction temperature and 50 Bar. It is doubtful that the overall process described is able to sequester more Carbon Dioxide as a metal carbonate than is generated as a consequence of manufacture of the alkali-metal hydroxides consumed in the process, or that the process is any cheaper to operate than Carbon Dioxide sequestration processes described in the prior art.
Therefore there is an urgent need to develop a process which is energy efficient, has a high throughput and is more cost effective than the prior art so that it can be used industrially.
From the foregoing, it will be appreciated that the known methods of processing and the systems for mineral Carbonation and Carbon Dioxide (CO2) sequestration, are neither optimal in their manner of operation nor adaptable to broader applications in a cost effective manner.